In drilling a well, it is conventional to employ a drill bit at the lower end of a drill string. The drill bit is urged against the formation to be drilled and the drill string is rotated to form the well. As the well becomes deeper, additional sections may be added to the drill string from the surface.
In drilling a well, and particularly a deep well, the diameter of the drilling string is very small relative to its length. Consequently, the longitudinal stiffness of the drill string is minimal. The drill string must constantly be kept in tension to prevent buckling. A heavy collar will typically form the lower part of the drill string which has a wall thickness relatively larger than the drill pipe to provide this tension.
The drill bit frequently tends to deviate or depart from straight vertical drilling. The deviation may result from a dip in the formation being drilled or a change in the formation characteristic and is amplified by the minimal longitudinal stiffness of the drill string. In some instances, the lower portion of the drill collar is severly buckled which results in a greater side force being transmitted to the drill bit. Even the collar, which may be several hundred feet in length, will buckle and provide inadequate stiffness and guidance to the drill bit.
Any of these factors or combination thereof may result in an unacceptable rate of well bore deviation. Well bore deviation occurs in every well drilled. A severe deviation can result in a "dog leg" which is a term used in drill operations to denote a deviation beyond the permissible limit.
Well bore deviation has always been a significant problem. With the increase in deep wells in offshore drilling, this problem assumes an even greater importance. Deviation may cause crooked holes, casing wear, key seating, stuck pipe and even breakage of the drilling string. This results in expensive fishing operations and, in a severe case, may cause a total loss of the well.
In an attempt to eliminate well bore deviation, stabilizers have been developed for use on the drill string. A near bit stabilizer is used directly above the drill bit to support the side loads generated by the drill collar buckling to support its length against the well bore formation and reduce the buckling of the drill collar. Stabilizers may also be positioned about the drill collar along its length for support against the well bore formation.
At the present time, stabilizers are either integral with the drill collar or have replacable sleeves or pads. The integral stabilizer may be machined in a short segment of a drill collar or formed by welding blades to the drill collar. These methods are quite uneconomical as the stabilizer must be discarded from the drill string or sent to the shop for redressing when worn.
The existing replacable stabilizers are secured either by threaded connections or compression fit. When a compression fit is employed, the stabilizer requires a hydraulic press for installation and removal. In addition, very close tolerance in machining operations and seal elements are necessary which delay both the installation and replacement on the rig floor and increase the cost of operation.
In one threaded stabilizer, internal threads are machined on the inside of the stabilizer sleeve. A second, matching thread is machined on the outside peripheral of the carrying sub or segment of the drill collar. A third threaded element is machined on the end of the tapered carrying sub which engages and tightens a third element of the stabilizer to achieve the restraint of lateral movement in the stabilizer sleeve. This type of stabilizer results in either a weaker element in the drill collar string if the inside diameter of the drill collar is maintained constant, or restricts the annular flow area through the drill string for flow of circulating fluid if sufficient material thickness is retained after machining the threads or producing a surface sufficient for compression fit on a stabilizer sleeve.
U.S. Pat. No. 4,245,709, issued Jan. 20, 1981 to Manuel illustrates one stabilizer sleeve secured by a friction fit. An outer stabilizer sleeve is threaded over a yieldable sleeve member placed over the drill collar. Cooperating threads between the member sleeves affect contraction of the yieldable sleeve member to secure the stabilizer to the drill collar periphery. This device is not wholy satisfactory. The required machining of the two elements of the stabilizer sleeve with internal and external cooperating threads result in a prohibitive cost for the final product. The depth of the circulation grooves between the stabilizing ribs is reduced to provide sufficient thickness for the external and internal thread machining. Finally, the drilling forces may cause the two elements of the sleeve to unthread, destroying the effectiveness of the stabilizer.
U.S. Pat. No. 3,818,999 issued June 25, 1974 to Garrett discloses a stabilizer having replacable wear pads. The device also has several shortcomings. The well bore annular area is restricted due to the geometry of the stabilizer body. An interruption of the circulating fluid and cutting flow is created by the rotation of vertical linear contact segments. More seriously, the loss of a contact segment in the well bore with integral tungsten carbide or hard metal elements may create severe problems in additional drilling operations. The hard metal may not be milled or removed readily.
Therefore, a need has arisen to provide a replacable sleeve stabilizer which is easy to replace on the rig floor. In addition, the stabilizer should be safer and more effective to use in the well and more economic to manufacture. Finally, the stabilizer should be readily replacable in the field with tools presently available on the rig site.